Concrete is the most commonly used man-made construction material in the world. Concrete includes a binder component and an aggregate component. The binder component is cement, which is generally formed of a calcined limestone-based composite, and the aggregate component is generally formed of quartz sand or calcium carbonate.
Improvements in the properties of concrete have been obtained in the past by combining the concrete with modifying additives. Common modifications to improve concrete include the addition of fibrous materials to the binder such as metal, polymeric, glass, and natural fibers, and formation in conjunction with rebar. Synthetic fibers have been used for several decades as a reinforcing agent for concrete, particularly for slab on grade applications. Fiber reinforced concrete can exhibit decreased shrinkage, decreased permeability and even increased abrasion and shatter resistance, depending upon the specific materials used in the composite.
The nature of the fiber reinforcement can vary. Microfibers, ranging from about 1 to about 10 denier are typically used to help prevent plastic shrinkage cracking as the concrete sets. The presence of the microfibers at loadings of about 1 to 3 pounds per cubic yard (lb/yd3) of concrete prevents micro-cracking of the concrete in the first 24 to 48 hours after pouring, as the bulk of the water in the mixture evaporates. Fibrillated or embossed macrofibers in the range of about 1000 denier are often added at loadings of 3 to 8 lb/yd3 as a secondary reinforcement. These fibers are added to improve overall toughness, as quantified by measurements of residual strength after first break on concrete samples containing the fibers (e.g., as measured according to ASTM C-1399). In both cases, the fibers ideally can be easily mixed with the wet concrete mixture and resist separating during the finishing and setting steps. Level of property enhancements to the concrete depends on both the strength of the fiber and adhesion between the fiber and the concrete matrix.
Polypropylene has been the material of choice by the concrete industry for both micro- and macrofibers. Polypropylene fibers can be easily formed via melt spinning (both micro- and macrofibers) or cutting from thin films (e.g., tape fibers or fibrillated fibers). Polypropylene fibers can exhibit tenacity on the order of 5 grams-force per denier (g/den). In addition, polypropylene is alkali resistant, which is critically important for any concrete additive (pH of concrete is typically 11 or higher). However, polypropylene does have disadvantages. Its low density and hydrophobicity combine to make it tend to bloom to the surface during finishing. This can cause surface appearance problems. Further, polypropylene fibers will not chemically bond to concrete, and rely only on mechanical interactions for adhesion to the matrix.
Other materials have been tried in an attempt to mitigate the disadvantages of polypropylene fiber additives. Polyamide fibers have been examined as polyamide is a denser material and thus expected to resist surface bloom. However, the moisture absorption of polyamides, resulting in lower strength and modulus, rendered these fibers less effective overall in concrete applications. Polyvinyl alcohol (PVA) fibers have also been developed for use in concrete. The obvious advantage is the potential for chemical bonding between the concrete matrix and pendant —OH groups on the polymer backbone. However, this sought-after bonding actually led to additional problems. In fact, pretreatment of PVA fibers with formaldehyde (HCHO(aq)) to bind a fraction of the —OH groups as the cyclic formal was found to be necessary to reduce the fiber—concrete interaction and reduce stress in the cured concrete product. In addition, PVA fibers are quite expensive and successful utilization requires an on-site, multistep mixing process with the concrete. Despite these difficulties, PVA fibers have found limited use in specialty concrete applications, such as precast concrete for earthquake-proof structures. Its use beyond these specialty applications has been quite limited.
While there have been improvements in composite materials incorporating fibrous polymeric materials, there remains room for further improvement and variation within the art. Fibrous polymeric modifiers and methods of using the modifiers with concrete that can provide further structural improvements to construction materials would be beneficial.